1. Field of Invention
This invention relates generally to a metal cooking pan and skirt assembly in which a heating unit within the skirt provides heat to cook food placed in the pan, and more particularly to an assembly of this type that includes means to constrain warpage of the assembly in the course of cooking and possible damage thereto.
2. Status of Prior Art
The coefficient of thermal expansion of any metal is the increment of a unit of length for a rise in temperature of one degree Fahrenheit. Hence a metallic structure will expand and contract as the temperature to which the structure is subjected rises and declines. The degree of expansion and contraction depends on the nature of the metal, for some metals have a relatively low coefficient of expansion while others have a higher coefficient.
Thermal stress is mechanical stress induced in a structure when not all of its parts are free to expand or contract to an equal degree in response to thermal changes. In most continuous structures, expansion and contraction cannot occur freely in all directions because of its geometry, various constraints and the existence of temperature gradients. Where the structure is a cooking utensil, thermal stresses produced in the course of heating may permanently deform and damage the utensil.
The reason for such deformation is that the thermal stresses produced in the course of a cooking operation cause the utensil to warp or twist, and if the stresses exceed the elastic limit of the metal, then the utensil will become permanently deformed. By definition, the elastic limit is the maximum stress a solid can sustain without undergoing permanent deformation.
The concern of the present invention is with a metallic structure that includes a rectangular skillet or pan for cooking or braising meat or other foods. In a known structure of this type, the rectangular pan is mounted above a skirt within which is disposed a gas or electric heating unit which applies heat to the base of the pan.
In the course of a cooking operation, the rectangular metal pan will be subjected to thermal stresses that may so warp and distort its structure as to permanently damage the pan. It is known to provide a pan of this type which is resistant to warpage. In this pan the working surface or base to which heat is applied takes the form of a thick steel plate that is welded to a thinner metal shell, the plate having a thickness of greater than a half inch. This steel plate, though resistant to warpage, makes the pan heavy and bulky, and also makes it difficult to handle.
Moreover, while heat is normally transferred from the plate at the base of the pan to meat or food resting on the plate, in some cases, particularly in a busy restaurant or hotel kitchen, the pan may inadvertently be heated for a prolonged period without there being in the pan any meat or food to absorb the heat. In that event, the base of the pan becomes overheated and the resultant warpage may irreparably damage the pan.
As pointed out in the Coudeurier U.S. Pat. No. 5,357,850, when the base of a frying pan is exposed to intense heat, the pan tends to warp. But while one could reduce the extent of such warpage by using a thicker metal for the base, this not only increases the cost of the pan but it also lengthens the cooking time, for a thick metal base reduces the rate of heat transfer to the food placed in the pan.
In Coudeurier, in order to prevent warpage, the base of the pan is formed by an array of radial grooves running from the center to the periphery of the pan to resist deformation of the base when it is subjected to intense heat.
In the Levin U.S. Pat. No. 3,045,664, to resist warpage of a cooking pot at elevated temperatures, the pot is provided at its bottom with a composite disc formed of a sheet of copper and a sheet of aluminum. In Faulk U.S. Pat. No. 1,730,030, the cooking utensil is provided with a base which is a laminate of aluminum and a dissimilar metal, such as iron or nickel, to prevent the utensil from warping. And in Bringham U.S. Pat. No. 1,201,138, a circular griddle is provided with spiral ribs at its base to strengthen the griddle and prevent warping thereof.